The present invention relates to an arrangement for control of exhaust pressure pulsation of a combustion engine.
The diesel engine is known for operational reliability and low fuel consumption but does not produce as low emissions as, for example, a petrol engine provided with a three-way catalyst. One way of improving the emissions from a diesel engine is to fit a particle filter which filters soot and particles from the exhaust gases and/or the NOx post-treatment system. These filters are usually very effective and gather both large and small particles. To prevent the filter from becoming full of soot and causing a major pressure drop for the exhaust gases leaving the engine, the soot has to be burnt. One method is for this soot to be burnt by the nitrogen oxides contained in diesel exhaust gases. In that case, the portion of the nitrogen oxides that takes the form of NO2 can oxidise the soot within the temperature interval of about 250 to 400° C., but this process takes a relatively long time and needs to be more or less constantly active even if there is an oxidation catalyst before the filter or the filter itself is covered with a catalytic layer. Another method for oxidising soot accumulated in the particle filter is to heat the filter to about 600-650° C. so that the surplus oxygen O2 from diesel combustion can oxidise the soot directly, which is a rapid process. Soot accumulated over many hours of operating time can be oxidised away in a time of the order of 5 to 10 minutes. The exhaust temperature of a diesel engine normally never reaches 600-650° C., particularly after a turbo unit whereby the turbine extracts power from the exhaust flow and causes a temperature drop. It is not unusual that the exhaust temperature after the turbine of the turbo unit is often lower than 250° C., a temperature (tit which an oxidation catalyst does not function. There are various special measures for temporarily increasing the exhaust temperature of a diesel engine.
Regenerating a NOx trap or a NOx catalyst requires hydrocarbon, e.g. in the form of fuel which can for example be supplied by post-injection into one or more cylinders.
Some of the most modern diesel engines are often equipped with exhaust gas recirculation (EGR) to reduce emissions of nitrogen oxides. Combining this system with particle filters and/or NOx post-treatment by so-called NOx trap or NOx catalyst entails complications. When it is desired to adopt measures intended, for example, to increase the temperature of the exhaust flow, there is no point in increasing the exhaust temperature in the EGR flow, since it has to pass through the EGR cooler, whereupon the increased exhaust heat increases the load on the EGR cooler without being used in any positive way but resulting rather in the fuel consumption being somewhat increased due to the increase in the exhaust temperature. There are therefore gains to be made with regard to fuel consumption by channelling increased exhaust temperatures to the exhaust system but preventing this temperature increase in the exhaust gases being recirculated in the form of cooled EGR. A deliberate increase in the exhaust temperatures of the various cylinders can be achieved, for example, by ordinary combustion being delayed and/or post-injection being effected in such a way that the extra fuel added is burnt but only contributes to a small extent to the expansion work in the cylinders.
The temperature of the exhaust flow before the particle filter can be increased to 600-650° C. by oxidising (burning) fuel in the oxidation catalyst fitted before the filter. This fuel can be injected in the exhaust line just before the catalyst, or extra injection can be effected in the cylinder, but within a crankshaft angle interval in which the conditions for the fuel to ignite in the cylinder are not fulfilled. This is for example the case when the fuel is injected into the cylinder during the latter part of the expansion stroke or during the exhaust stroke, so-called late post-injection. If the fuel is injected into the cylinder, the same injection equipment can preferably be used as for the ordinary fuel injection. This avoids the cost and complications of a further extra injector for the fuel which is to be oxidised in the oxidation catalyst. If the extra fuel is supplied by late post-injection in all the cylinders and the engine at the same time uses feedback of exhaust gases to the inlet (EGR), this means that part of the unburnt fuel intended for the catalyst reaches the EGR circuit and the inlet ducts. Any fuel vaporised when leaving the cylinders during the exhaust stroke but later cooled in an EGR cooler can be condensed and part of the resulting liquid may possibly accumulate in recesses/pockets during certain modes of operation. When the engine's operating mode subsequently changes and the gas flow changes, the liquid accumulated may possibly accompany the gas flow momentarily and immediately enter one or more cylinders in an uncontrolled manner. This gives rise to uncontrolled behaviour of the engine and may cause serious accidents and/or damage. Fuel which does not condense but passes through the EGR cooler and the inlet ducts in a vaporised form also gives rise to altered combustion conditions. In a diesel engine, the fuel should in fact not arrive at the same time as the inlet air, as the fuel supply timing should be determined by the injection system. Even a small leak in the EGR circuit may become very obvious if condensed fuel begins to trickle out through the leak.
These adverse effects due to fuel in the EGR circuit can be prevented either by closing the EGR circuit when late post-injection takes place, or alternatively, as a consequence of the NOx level increasing from the combustion, applying late post-combustion only in carefully selected cylinders in combination with designing the exhaust manifold in such a way that EGR gases are only taken from cylinders which have no late post-injection. U.S. Pat. No. 5,987,884 and U.S. Pat. No. 6,141,959 describe how an exhaust system divided into two portions, combined with late post-injection in certain designated cylinders, to avoid the above problems caused by fuel in the EGR circuit. The method of dividing an exhaust system into two portions usually means that the exhaust system is more expensive to manufacture and requires more space for installing it.
It is desirable to provide an arrangement in a combustion engine which avoids the effects on the flow in the EGR circuit which are caused by means for regeneration of an exhaust post-treatment unit situated in the exhaust system even in the case of exhaust systems in which all the exhaust gases are collected in one and the same duct entirely outside the exhaust ports of each cylinder. It is also desirable to provide uniform pressure pulses at both the exhaust outlet of the exhaust manifold and its EGR outlet.